Structure-function studies on the AT1 receptor, which mediates most of the known physiological actions of angiotensin II (Ang II), have revealed further aspects of the domains involved in receptor activation and signal transduction. In addition to a conserved tyrosine residue (Tyr-215) in the 5th transmembrane domain, a conserved apolar amino acid (Leu-222) in the amino-terminal region of the third cytoplasmic loop was found to be essential for the activation of the AT1 receptor by Ang II. The Lys-222 mutant receptor was unable to adopt the high-affinity binding conformation, did not couple to G proteins, and did not mediate activation of phosphoinositide hydrolysis. A search revealed that most G protein-coupled receptors contain an apolar residue in this position, suggesting that this amino acid is an essential element in the agonist-induced activation not only of the AT1 but possibly many other such receptors. The AT1a and AT1b receptor subtypes expressed in stably transfected adrenal Y1 cells exhibited small but significant differences in their binding pharmacology, and were coupled through a pertussis-insensitive Gq-type protein to the phosphoinositide/calcium signaling pathway. However, in contrast to their behavior in several Ang II target tissues, neither subtype was coupled through Gi to inhibition of adenylyl cyclase when expressed in murine adrenal tumor cells or COS cells. An orphan receptor was isolated in the course of cloning the angiotensin AT1b receptor and was subsequently identified as the adrenomedullin receptor, based on its binding pharmacology, tissue distribution, and functional properties. The identification of this receptor cDNA should facilitate investigation of the regulation and actions of adrenomedullin in a wide range of normal and neoplastic tissues. The recently identified cytosolic wortmannin-sensitive phosphatidylinositol 4-kinase (PI4K) was found to be distinct from the recognized membrane-bound type II PI4K, but shared several properties with the membrane-bound Type III PI4K of bovine brain. Thus, Type III rather than type II PI4K enzymes are responsible for the maintenance of the precursor phospholipid pools that are required for agonist-induced signaling through the inositol phosphate/calcium pathway. Studies on agonist-induced calcium transients in adrenal glomerulosa cells revealed that calcium influx through VSCCs modulates the frequency of the oscillatory calcium signals elicited by physiological Ang II concentrations. However, at higher agonist concentrations Ca2+ influx occurs predominantly through the capacitative entry pathway, and influx through VSCCs makes little contribution to the resulting non-oscillatory calcium signals.